1. Field
This disclosure relates to a radio frequency-lightwave (RF-lightwave) waveform generator capable of generating a set of frequency-spread and frequency-hopped or frequency modulated radio frequency (RF) waveforms. This disclosure also relates to the generation of multi-tone optical combs with a photonic oscillator that may be used with or as part of the waveform generator. This disclosure also relates to pre-processing of the frequency-spread and frequency-hopped or frequency modulated radio frequency (RF) waveforms. The generated waveforms may be further amplitude-modulated with a pulse code and may be used, for example, as transmit waveforms for a radar system. Pre-processing may be used on a receive waveform or radar return signal. This pre-processing can then effectively create multiple short pulses from a single long pulse and combine the information from those short pulses.
2. Description of Related Art
A multi-tone, frequency-hopped or frequency-modulated RF-lightwave waveform may function as a lightwave carrier for an optical transmission channel. The RF signal information carried by the optical transmission channel may be a pulse code, for example, which may be imposed onto the multi-tone, frequency-hopped or frequency-modulated RF-lightwave carrier by means of a lightwave modulator. The final RF-lightwave waveform can be transmitted (by means of an optical fiber link or a free-space optical link) to a photoreceiver. The photoreceived signal, which is in electronic form (frequency converted and demodulated), can then be transmitted through a RF channel (an antenna or wireless link).
Multi-tone frequency spreading may be used to make the resultant signal difficult for a non-coherent receiver to detect. Use of a frequency-spread carrier is one method to produce a signal that has Low Probability of Interception (LPI) by conventional intercept receivers. In addition, if the precise frequency of the carrier can be changed and is unknown to the interceptor, LPI performance is enhanced. These techniques are useful in LPI radar and communication systems.
Typically, an interceptor would use a wideband receiver that is channelized into smaller frequency bands to detect and identify the signal. If the signal falls within a single channel of the receiver, then it can be detected. However, if the signal is spread in frequency so those portions of it fall within many channels, it is difficult for the interceptor to distinguish that signal from the background noise. Typically, the channels of the intercept receiver may be scanned or long integration times may be used to sense an incoming signal. If the signal frequency is varied rapidly to move between different channels within the sensing time, it also appears like noise. Alternatively, if the signal frequencies are varied rapidly with time although those hops lie within the received channels, that signal will be detected but difficult to identify. The dense, multi-tone waveforms that can be generated by periodic frequency modulation can be designed to have a spacing that is smaller than the multi-tone waveform produced by the photonic oscillator to allow intercept receivers having smaller channel spacings to be defeated.
Another purpose of frequency spreading is to make a signal less susceptible to jamming. The frequency coverage of the jammer may not be as large as the coverage of the frequency-spread carrier. In addition, since the frequency-spread carrier consists of discrete tones that can be summed coherently, the signal power is used more efficiently. This is in contrast to the jammer, which is uniformly broadband. Rapid switching of the signal band also makes it less susceptible to being jammed, since the jammer cannot predict from one signal pulse to the next which frequency to jam.
Previous methods to achieve LPI performance are based on using electronic synthesizers to produce the waveforms. Typically, a pulse-compression code is used to phase modulate a single-tone carrier and spread the spectrum. For example, if the signal pulse is 1 μsec wide and a 100-to-1 pulse compression code is used, a signal bandwidth of 100 MHz is obtained. The channel bandwidth of the interrogating receiver is typically much narrower than this. The bandwidth of present high-dynamic-range analog-to-digital converters is typically 100 MHz or less. Thus, interrogator channel bandwidths are also 100 MHz or less. Thus, being able to produce signals with a frequency range much greater than 100 MHz allows for such interrogators to be defeated. Methods known in the art to generate multi-tone wavelengths may use electronic synthesizers to produce the waveforms. Multiple separate electronic synthesizers may be used, but only a small number of tones may be produced to cover large bandwidth. Alternatively, frequency tunable electronic synthesizers may produce a waveform having a Fourier spectrum that contains a larger number of tones, but over a much smaller bandwidth. Gradual modulation or chirping of the frequency of a single tone waveform for the purpose of pulse compression is known in the art. Frequency modulation to compress a single tone pulse is common and large frequency excursions and pulse-compression ratios are possible. However, frequency modulation to compress a multi-tone pulse has been difficult to accomplish. Frequency modulation of a large number of tones has previously been cumbersome and expensive to implement. Hence, an apparatus and method that provides for frequency modulation of a large number of tones with less complexity and/or expense would be desirable.
LPI waveforms typically have low instantaneous transmitted power. In order to increase the transmitted power, a longer radar pulse may be used. However, long pulses have poor resolution of the target range, since returns that occur within a given pulse duration are not distinguishable. Methods are known in the art for processing FM waveforms for achieving pulse compression and improved range resolution. Some of the known methods mix the Transmit and Receive waveforms to compare them. However, a priori knowledge of the approximate target range may be needed to synchronize the two waveforms in these known methods. Therefore, an apparatus and method that does not require a priori knowledge of the approximate target range would be desirable for the determination of the actual range.
Prior art methods to process multi-tone waveforms typically involve using a bank of filters to spectrally separate those tones, which then can be processed individually. Usually, these methods can accommodate only tones of fixed frequency. Hence, an apparatus and method that can process variable frequency, frequency-modulated, multi-tone waveform would be desirable.
The prior art includes:
1. A single-tone, single-loop optoelectronic oscillator—see U.S. Pat. No. 5,723,856 issued Mar. 3, 1998 and the article by S. Yao and L. Maleki, IEEE J. Quantum Electronics, v.32, n.7, pp. 1141-1149, 1996. A photonic oscillator is disclosed (called an optoelectronic oscillator by the authors). This oscillator includes a single laser and a closed loop comprised of a modulator, a length of optical fiber, a photodetector, an RF amplifier and an electronic filter. The closed loop of this oscillator bears some similarity to the present invention. However, the intent of this prior art technique is to generate a single tone by incorporating an electronic narrow-band frequency filter in the loop. A tone that has low phase noise is achieved by using a long length of the aforementioned fiber. Demonstration of multiple tones is reported in this article achieved by enlarging the bandwidth of the filter. However, the frequency spacing of those multiple tones was set by injecting a sinusoidal electrical signal into the modulator. The frequency of the injected signal is equal to the spacing of the tones. This method causes all of the oscillator modes (one tone per mode) to oscillate in phase. As a result, the output of this prior art oscillator is a series of pulses. See FIG. 14 (b) of this article.2. A single-tone, multiple-loop optoelectronic oscillator—see U.S. Pat. No. 5,777,778 issued Jul. 7, 1998 and the article by S. Yao and L. Maleki, IEEE J. Quantum Electronics, v.36, n.1, pp. 79-84, 2000. An optoelectronic oscillator is disclosed that uses multiple optical fiber loops, as the time-delay paths. One fiber loop has a long length and serves as a storage medium to increase the Q of the oscillator. The other fiber loop has a very short length, typically 0.2 to 2 m, and acts to separate the tones enough so that a RF filter can be inserted in the loop to select a single tone. The lengths of the two loops, as well as the pass band of the RF filter, can be changed to tune the frequency of the single tone that is generated. This approach teaches away from the use of multiple optical loops to obtain multiple tones, since it uses the second loop to ensure that only a single tone is produced.3. 1.8-THz bandwidth, tunable RF-comb generator with optical-wavelength reference—see the article by S. Bennett et al. Photonics Technol. Letters, Vol. 11, No. 5, pp. 551-553, 1999. This article describes multi-tone RF-lightwave comb generation using the concept of successive phase modulation of a laser lightwave carrier in an amplified re-circulating fiber loop. The lightwave carrier is supplied by a single input laser whose optical CW waveform is injected into a closed fiber loop that includes an optical phase modulator driven by an external RF generator. This results in an optical comb that has a frequency spacing determined by the RF frequency applied to the phase modulator and absolute frequencies determined by the wavelength of the input laser. The loop also contains an Er-doped optical fiber amplifier segment that is pumped by a separate pump laser. The effect of the optical amplifier in the re-circulating loop is to enhance the number of comb lines at the output of the comb generator. One may expect some mutual phase locking between the different comb lines since they are defined by the phase modulation imposed by the external RF generator. This approach does not include a photodetector in the loop.4. One technique for generating a RF signal is by optical heterodyning. See FIG. 1. With this technique, as shown in FIG. 1, the optical outputs of two laser wavelengths produced by a RF-lightwave synthesizer are combined onto a photodetector. In a simple example, the RF-lightwave synthesizer may consist of two lasers each producing single wavelengths, i.e., a single optical frequency, spectral tone, or single spectral line. When the combined output of the two lasers is converted by a photodetector into an electronic signal, that electronic signal has frequency components at the sum and difference of the two laser lines. The photodetector output is proportional to the incident optical power (or the square of the electric field of the incident light). The sum frequency is a very high optical frequency, while the difference frequency is typically in the RF range. The photodetector may also be used with electronic bandpass or low-pass filters or the photodetector itself may act as a low-pass frequency filter so that only the heterodyne difference frequency in the desired range of frequencies is output from the photodetector as a current or voltage.
In order for the heterodyne output produced by the photodetector to have low phase noise, the two laser lines must be locked together, so that their fluctuations are coherent. Various methods known in the art can be employed to achieve this locking. One technique is to optical-injection lock both lasers (typically referred to as slave lasers) to different phase-locked tones (or spectral lines) that are emitted by a third laser (typically referred to as a master laser).
Optical heterodyning can be combined with an external optical modulator to perform frequency conversion (frequency translation). This function is also illustrated in FIG. 1. A dual-line lightwave output of an RF-lightwave synthesizer (such as one or both of the tones that are output from the two slave laser discussed above) is supplied to an optical intensity modulator, with a typical modulator being a Mach-Zehnder interferometer. A RF input signal may also be supplied to the modulator, which applies an intensity modulation onto the lightwave signal. The transfer function of the modulator results in the generation of frequency sum and difference terms. The output of the photodetector is another RF signal with frequency components that are the sum and difference between the frequencies of the RF input ωRF and the frequency spacing between the two laser lines. In essence, the frequency difference ωLO of the two laser-lines acts as a local-oscillator (LO) frequency that is multiplied with the RF input signal to produce an intermediate frequency (IF) ωLO−ωRF. A mathematical expression for this process is given as:
      i    D    =                    α        ⁢                                  ⁢                  I          o                            2        ⁢                  L          MOD                      ⁢                  ⁢          {              1        +                  m          ⁢                                          ⁢                      sin            ⁡                          (                                                ω                  RF                                ⁢                t                            )                                      +                              M            ⁢                                                  ⁢                          cos              ⁡                              (                                                                            ω                      LO                                        ⁢                    t                                    +                  ϕ                                )                                              ±                                    1              2                        ⁢            mM            ⁢                                                  ⁢                          sin              ⁡                              [                                                                            (                                                                        ω                          LO                                                ±                                                  ω                          RF                                                                    )                                        ⁢                    t                                    +                  ϕ                                ]                                                        }      where iD is the photocurrent output from the photodetector.5. A Brillouin opto-electronic oscillator described by Yao in U.S. Pat. No. 5,917,179 issued Jun. 29, 1999. The oscillator disclosed by Yao produces a single tone rather than multiple tones. Further, the oscillator makes use of stimulated Brillouin scattering (SBS) in an optical fiber in the opto-electronic feedback path of the oscillator. This feedback path may have one or more optical and/or electrical loops. The SBS of light provided by a pump laser produces a second optical signal that also is fed to the photodetector in the path. The frequency of this second signal is used to define the frequency of the electrical drive signal for the optical modulator of the path. The photodetector produces an electrical signal that is the beat of the SBS-produced and the modulator-output optical signals. This beat signal is used to drive the optical modulator and create another modulated output signal that is fed into the optical fiber exhibiting SBS. This approach uses SBS of the light generated by a pump beam. It does not uses SBS of light derived from the output of the optical modulator.